1. Field of the Invention
The present invention relates to image processing for embedding additional information in an image.
2. Description of the Related Art
Techniques for embedding information related to an image in image information are being actively studied. In recent years, a so-called digital watermarking techniques has been standardized. With this technique, upon distribution of image information of a photograph, a picture or the like via, e.g., the Internet, additional information such as an author name and an authorized/unauthorized state is embedded in the image information such that the additional information cannot be visually discriminated.
Further, in a method using density patterning, additional information is embedded by utilizing a code book where an output pattern is set in correspondence with luminance. That is, an output pattern corresponding to a code to be embedded is recorded in the code book for a particular luminance. This code book is used for determining an output pattern corresponding to a code to be embedded, and additional information is embedded in an image region.
Further, in a method using ordered dithering, dither matrices A and B are utilized, and a threshold value matrix is changed in correspondence with additional binary information. In the matrix B, adjacent row components in the matrix A are exchanged. A matrix is selected in correspondence with additional information to be embedded, and an image for release and an image for verification are generated. When the image for release and the image for verification are overlapped with each other, the embedded additional information can be extracted.
Further, in a method utilizing ordered dithering, additional information is embedded by utilizing a deviation of correlation in an arbitrary-sized dither matrix. A coefficient of a low-correlation dither matrix is changed in correspondence with a code to be embedded. At this time, an image is divided into blocks by dither matrix size, and a code is embedded in a block. The block where the code is embedded is held as a private key.
Further, in a method utilizing pixel substitution, additional information is embedded by substituting the additional information with bits on a particular bit plane. The bit plane where embedding processing has been performed and the pixel positions are held as a private key. Further, the image information and the additional information may be exchanged not in the pixel position but in a bit stream. Further, the result of an exclusive OR between the additional information and a shift register series may be embedded.
Further, as a method utilizing pixel substitution, the patchwork method is known. In this method, an image is divided into blocks of an arbitrary size, and a variance value for correlation among arbitrary pixels in a block is shifted in correspondence with additional information.
Further, in a method utilizing frequency transformation, additional information is embedded in a high frequency component of image data that has been transformed to frequency component data by discrete Fourier transformation. The position where the additional information has been embedded is held as a private key.
Further, in a method for embedding additional information in a JPEG image, a transformation coefficient in a discrete cosine transformation is manipulated. A particular coefficient value in each divided block is changed in correspondence with the additional information, thereby the additional information can be embedded.
Further, in a method for embedding additional information in a color image, additional information is embedded only in a particular color plane among RGB colors. By utilizing the above-described density pattern method, the pixel substitution method, or the like, the additional information is embedded in a particular plane.
Further, as an application of digital watermarking, forgery prevention or forgery tracking is known. By utilizing the technique, unauthorized duplication (forgery) of bank notes, stamps, marketable securities and the like in accordance with the availability of high-quality image output devices such as a copier and a printer can be prevented. That is, additional information is embedded in an image so as to specify a serial number or the like of an image output device from an image printed on a paper sheet.
Further, in a method utilizing error diffusion, additional information is embedded by periodically changing a quantization threshold value when performing error diffusion processing, in correspondence with additional information to be embedded, thereby forming a modulated periodic pattern in an image. Further, in a method utilizing a particular ink color for embedding additional information, a pattern corresponding to the additional information is embedded in a particular ink color plane, and thereby the additional information is embedded in a print image. Note that image information processed for a digital watermark using the above-described density pattern method, pixel substitution method, or frequency transformation method may be output.
Regarding these various digital watermark embedding methods, to solve problems such as robustness improvement upon extraction of additional information and improvement of the quality of an image including additional embedded information, the following methods are proposed.
Japanese Patent Application Laid-Open No. 2003-101760 discloses, upon embedding additional information in an image region, selecting a method for embedding the additional information based on spatial coordinates of a print medium, in consideration of printing differences due to precision of a printing device. Further, Japanese Patent Application Laid-Open No. 2003-101760 discloses selecting a method for extracting the additional information based on the spatial coordinates upon reading an original, in consideration of differences in the precision of an extractor.
Japanese Patent Application Laid-Open No. 2000-301932 discloses, upon performing image formation by repeating printing via repetitive main scanning and subscanning of a print head of a printer and a print medium, embedding additional information in a part of the main scanning processing, and not embedding the additional information in another part of the main scanning processing. By this arrangement, degradation of performance of additional information extraction due to partial image overlap in a subscanning direction can be prevented.
Japanese Patent Application Laid-Open No. 2002-354227 discloses analyzing an image in which additional information is to be embedded by region, and based on the analysis result, changing the amount of additional information to be embedded by region, in correspondence with a visual digital watermark recognition level.
However, in the technique of changing the method for embedding based on the spatial coordinates of the print medium, as disclosed in Japanese Patent Application Laid-Open No. 2003-101760, when the same code is consecutively embedded, pattern consecution occurs and the image quality is degraded.
Further, in the technique of embedding the additional information only in a particular part of the main scanning processing, as disclosed in Japanese Patent Application Laid-Open No. 2000-301932, when the same code is consecutively embedded in a particular part of the main scanning processing, pattern consecution occurs and the image quality is degraded.
Further, in the technique of analyzing and changing the embedding processing by image region disclosed in Japanese Patent Application Laid-Open No. 2002-354227, when the same code is consecutively embedded, pattern consecution occurs and the image quality is degraded.
Note that in methods other than the above-described digital watermark embedding methods for embedding a particular pattern in an image region, when additional information in which the same code consecutively appears is embedded, the same embedding processing is repeated. This causes low frequency noise and degrades image quality.